Enforce Python 2.5 for dev envs
Enforce Python 2.5 for dev envs

The oldest version of Python we support is Python 2.5. We should develop
against that because we can trust that newer Python versions of Python
will be backwards-compatible. By developing with the oldest version we
make it less likely that we'll break compatibility.

gratipay/aspen.py,gratipay/aspen.py